Pin view options button to leading toolbar edge on macOS

Place the view options control on the leading (`.navigation`) edge on
macOS so it is consistently the first/leftmost toolbar button across all
views, instead of sitting on the right next to the search field in
search views and on the left in non-search views.

Each affected view gains a computed `viewOptionsPlacement` (`.navigation`
on macOS, `.primaryAction` elsewhere) so iOS/iPadOS/tvOS keep the
button trailing. Search-style views (Search, InstanceBrowse) declare the
view options item first so it renders left of the search-filters button;
their flexible spacer is kept to right-pin the search field. Channel's
fixed spacer between view options and the channel menu is now iOS-only.

Yattee/Views/Downloads/DownloadsView.swift

@@ -27,6 +27,15 @@ struct DownloadsView: View {
         appEnvironment?.settingsManager.listStyle ?? .inset
     }
 
+    /// View options control lives on the leading edge on macOS, trailing elsewhere.
+    private var viewOptionsPlacement: ToolbarItemPlacement {
+        #if os(macOS)
+        .navigation
+        #else
+        .primaryAction
+        #endif
+    }
+
     var body: some View {
         Group {
             if let manager = downloadManager, let settings = downloadSettings {
@@ -45,7 +54,7 @@ struct DownloadsView: View {
                 ToolbarSpacer(.flexible, placement: .primaryAction)
             }
             #endif
-            ToolbarItem(placement: .primaryAction) {
+            ToolbarItem(placement: viewOptionsPlacement) {
                 if let settings = downloadSettings {
                     sortAndGroupMenu(settings)
                 }